7.3 ford is pretty reliable, but they are hard to find in nice shape anymore.

6.0 fords can be made good, read bones post.

cummins motors are workhorses, the rest of the dodge sucks in my opinion, crew cabs aren't roomy unless you get the ugly mega cab.

Chevy's look the best and ride the best IMO, plus you can get the allison. Duramax seems like a good motor, certain years had injector issues, but then again so did some of the cummins and the 6.0 fords.

It's a coke/pepsi thing no matter which way you go. They all have bright spots and they all have issues. Determine what aspects are most important and then just buy a truck